Light-emitting diodes (LED) are semiconductor light sources traditionally used as indicator lamps in many devices. In addition, LEDs are increasingly also being used for lighting, where one particular use is for providing backlighting. For example, LED backlighting is increasingly being used for liquid crystal displays (LCDs), as LCDs do not produce their own illumination. Furthermore, LED backlight lighting systems are becoming increasingly common for the use in display backlighting and keypad backlighting in portable devices such as cell phones, smartphones, PDAs, digital cameras, personal navigation devices and other portable devices with keypads and/or LCD displays.
LED lighting systems are generally associated with a variety of advantages over traditional lighting sources such as incandescent lighting. For example, LEDs are efficient, associated with longer life, exhibit faster switching and produce less heat than traditional lighting sources. Due to the faster switching characteristics of LEDs, they are suitable for use in fast and highly responsive circuits by allowing for both quick response/start-up time and the capability to be operated at high frequency, further allowing for such enhancements as frequency modulation in order to reduce power consumption.
LED lighting systems typically comprise “strings” of stacked LEDs in which multiple LEDs are connected in series. Therefore the LED driver control circuit has to be able to provide a regulated high supply voltage. A common practice is to pull a well-defined current from the bottom of each LED string, via current sources or resistors and regulating the voltage across them. In such a way the power dissipation across the current sources can be minimized. In order to protect the system components from excessive voltage levels and avoid excessive high current to flow in the LED circuit, an overvoltage protection mechanism is generally provided to disable the delivery of power to the circuit in the event that the voltage rises above a certain threshold.
In lighting systems with a common voltage supply and various strings of LED similar to the one illustrated in FIG. 1, a short circuit between ground voltage and the cathode of one of the lowermost LED (feedback nodes 102-1 to feedback nodes 102-N in FIG. 1 below) causes a voltage drop at the affected LED string. In such a condition, the feedback mechanism employed to regulate the drive voltage provides no control over the current flowing to the LEDs of the affected LED strings. Such a short circuit condition generally causes the drive voltage to be further increased leading to an uncontrollable boost of the current flowing to the LEDs of the affected LED strings and to an increase of the heat generated by the system. If the short circuit remains for some time, the involved components can be damaged.
Thus, there is a need for a fault-tolerant controller that is capable of providing short circuit detection.
This application provides a controller for a lighting system comprising a plurality of light emitting diode “LED” circuits. A controllable power source provides a drive voltage to power the plurality of LED circuits. The controller comprises a control unit configured to cause regulation of the drive voltage based on a determination of a plurality of feedback voltages, one feedback voltage for each of the plurality of LED circuits.
The controller further comprises a fault condition detecting means configured to identify one or more fault conditions from the plurality of feedback voltages. A LED circuit is determined as a fault circuit having a fault condition if its respective feedback voltage is below a first fault-circuit condition threshold. The fault condition detecting means is further configured, in response to a detected fault condition, to apply a test voltage to a cathode of the fault circuit. If the feedback voltage of the fault circuit remains below a second fault-circuit condition threshold, the fault circuit is determined to have a short circuit condition. The fault condition detecting means may include a pull-up circuit that can apply a test voltage or test current between ground voltage and a cathode of the fault-circuit in an attempt to pull the node up that has been detected with a fault condition. However, if there is a short circuit, the feedback voltage cannot be pulled up, the detection of which indicates a short circuit condition. By way of example, the first fault-circuit condition threshold may be set to the same threshold value as the second fault-circuit condition threshold.
In accordance with a further aspect, the fault circuit may be determined to have an open circuit condition if the feedback voltage of the fault circuit exceeds the second fault-circuit condition threshold when the test voltage or test current is applied to the cathode of the fault circuit. Consequently, the controller described in this application may provide for both short circuit and open-string detection and can distinguish between these two conditions based on the same fault condition detecting means. For example, in the LED lighting systems with “strings” of stacked LEDs in which multiple LEDs are connected in series, if one of the many individual LEDs in an individual LED string fails, an open circuit condition for the entire associated LED string can occur. In such a condition the feedback mechanism employed to regulate the drive voltage generally causes the drive voltage to be further increased up to the point where the overvoltage protection circuitry would disable the entire lighting system. LED lighting systems can have many LED strings, for example five, six or even thirty or more. Consequently, if an open circuit condition occurs in any one of the LED strings, the entire lighting system becomes inoperable due to the overvoltage protection mechanism. While this solution does successfully protect the circuits from the excessive currents associated with an overvoltage condition, the entire circuit and all the LED strings become unusable if there is an open circuit condition in only one of the LEDs of one of the plurality of LED strings. The proposed fault-tolerant controller is configured to not only detect a short circuit condition but also an open-string condition and to distinguish between these two conditions which allows to continue operating the system in the event of an open circuit condition in one or more of the plurality of LED strings, e.g. by excluding the string having the open circuit condition from the voltage feedback control mechanism.
In accordance with a further aspect, the fault condition detecting means may comprise a plurality of pull-up circuits, one pull-up circuit provided between ground voltage and a cathode for each LED circuit. Each pull-up circuit may comprise a voltage source and a switch. The fault condition detecting means may be configured to close the switch associated with the fault circuit upon detecting a fault condition to apply the test voltage or test current between ground voltage and the cathode of the fault circuit. By way of example, the test voltage is small compared to the boost voltage to operate the LED strings in order not to damage the LEDs in case of a short circuit condition.
The controller may further comprise a plurality of comparators and a plurality of logic gates, one comparator and one logic gate for each LED circuit, to detect a fault-circuit condition indication for each of the LED circuits. A comparator may be configured to conduct a comparison between the respective feedback voltage of one of the plurality of LED circuits and the first fault-circuit condition threshold, and to input the comparison result into the respective logic gate. The use of a plurality of comparators allows the controller to determine a respective fault condition indication for each LED circuit. The feedback voltages are then compared to the first and/or second fault-circuit condition threshold.
In accordance with a further aspect, the fault condition detecting means may be configured to determine whether a short circuit condition exists in the plurality of light emitting diode “LED” circuits during a pre-start test before the normal operation of the LED lighting system is started. During the pre-start test no drive voltage is supplied to the plurality of light emitting diode “LED” circuits. A short circuit condition often results from error in the manufacturing process. Thus, testing the LED circuits for a short circuit condition before using the LED circuit under normal operational load and with no drive voltage being supplied to the plurality of light emitting diodes allows detecting such a condition before the LEDs are damaged by the short circuit condition. In order to protect the LEDs from uncontrolled current flows under a short circuit condition, the controller is preferably configured to disable the drive voltage and the boost voltage to power the plurality of LED circuits after a short circuit condition has been detected during the pre-start test. Preferably a LED circuit having a detected short circuit condition is disabled, i.e. switched off from the drive voltage provided by the power source, e.g. by blowing respective fuses in the LED circuit or setting/opening respective switches in the LED circuit. This allows usage of the lighting system under reduced conditions (e.g. reduced light emission) while neither components of the LED circuit nor the power supply is damaged during operation of the lighting system.
In accordance with a further aspect, in response to a pre-start test with no short circuit condition being detected, the controller may be configured to wait for a predetermined time interval before regularly determining during operation under a normal load condition whether a fault condition exists in one of the plurality of light emitting diode “LED” circuits. This has the advantage that a false detection during unstable and transient start-up conditions is avoided. The regular determination of a fault condition during normal load operation can be performed in regular time intervals, or after a predetermined time under normal load operation has passed, so as to periodically test the LEDs for being operative.
In accordance with a further aspect, the controller may further comprises a minimum voltage selector configured to exclude the one or more respective feedback voltages for each respective LED string associated with an open circuit condition, thereby excluding the respective feedback signal from the determination of the minimum feedback voltage. This improves the regulation of the drive voltage and allows the lighting system to continue to operate in the event of an open circuit condition in one or more of the plurality of LED strings.
In order to improve the detection accuracy of a fault condition, the fault condition detecting means may be configured to sample the feedback voltage of the fault circuit a predetermined number of times (e.g. at least two times) before determining the short circuit condition of the fault circuit.
In addition, a lighting system comprising a plurality of light emitting diode “LED” circuits, a controllable power source and the controller as described above is provided to provide LED short circuit detection and/or LED open circuit detection.
In addition, a method of detecting a fault condition within a plurality of light emitting diode “LED” circuits of a lighting system is provided. The method comprising the steps: determining feedback voltages for each of the LED circuits of the plurality of LED circuits; causing regulation of a drive voltage to power said plurality of LED circuits based on the determination of a plurality of feedback voltages; determining a fault circuit having a fault condition among the plurality of LED circuits for which the respective feedback voltage of the respective LED circuit is below a first fault condition threshold; in response to the detected fault condition, applying a test voltage to a cathode of the determined fault circuit having the fault condition, and if the feedback voltage of the fault circuit remains below a second fault-circuit condition threshold, determining the fault circuit as having a short circuit condition. Preferably a LED circuit having a short circuit condition is disabled, i.e. switched off from the drive voltage provided by the power supply so that neither components of the LED circuit nor the power supply are damaged during operation of the lighting system. By disabling a particular LED circuit having a short circuit condition it is possible to operate the remaining LED circuits of the lighting system without the need to disable or discard the entire lighting system.
The method may further comprise the step of determining the fault circuit as having an open circuit condition if the feedback voltage of the fault circuit exceeds the second fault-circuit condition threshold after the test voltage has been applied to the cathode of the fault circuit. In this case, it can be concluded that the fault LED circuit has an open circuit and should be excluded from providing a feedback voltage for the power regulation loop. Normally, a LED circuit with an open circuit will not draw power and does not otherwise disturb operation of the other LED circuits which can therefore still be used.
In accordance with a further aspect, the method of detecting a fault condition is conducted as a pre-start test of the lighting system before the normal operation of the LED lighting system is started. During the pre-start test no drive voltage is supplied to the plurality of light emitting diode “LED” circuits. The lighting system may be disabled in response to a detected short circuit condition during the pre-start test, or the only LED circuit having the short circuit is disabled while the other LED circuits are continued to operate.
The method may further comprise the steps of: enabling the lighting system after no short condition has been detected during the pre-start test, waiting for a predetermined time period before carrying out the steps for detecting a short circuit condition and a open circuit condition in predetermined time intervals; and in response to a detected open circuit condition, excluding the respective feedback voltage associated with the open circuit condition from said determination of a plurality of feedback voltages.
It will be appreciated that the method steps and apparatus features may be interchanged in many ways. In particular, the details of the disclosed apparatus can be implemented as a method, and the disclosed method steps implemented as apparatus features, as the skilled person will appreciate.
The invention is explained below in an exemplary manner with reference to the accompanying drawings.